An interesting exception to the stamping rules seems to have evolved
in the “Western” style jewelry in the United States - regarding hand
engraved belt buckles, conchos, and ornaments used on saddlery.

During the 30 years that I have been observing, making and engraving
these items - I would say that 99% of the “sterling” belt buckles
(This includes those of major manufacturers) are made with nickle
findings on the back side.

Conchos and saddlery ornaments have nickle, brass, or more commonly
copper findings soldered to the back side.

These items are also made with jewelers bronze overlays, figures,
and motifs on the front side - yet they are all stamped
"sterling"… if that is what the major material is.

None of these items - and there are literally billions of them out
there - would assay as sterling if they were melted…

To my knowledge there has never been any complaint made - nor has
the U.S. government ever intervened. All of this was customary long
before I entered the trade. (and, I suspect - before the U.S.
government stamping laws were put into effect, as I have seen
articles made in this way, stamped sterling, that are at least 100
years old.)

I just examined a box full of this kind of product, made in Mexico,
and imported into the U.S. This box went through customs, as do
hundreds every day. Every single item is stamped “sterling”. Every
single item has a base metal finding or findings soldered to it.
Some have jewelers bronze overlays.

This does not apply to the relatively new material known as “silver
overlay”, which is a laminate of sterling over nickle - and is
customarily stamped “Sterling Overlay”. This material has only been
around for 20+ years, and is generally used in the lower end, mass
produced product.